The present invention relates to an image forming device and an image forming method. More particularly, the present invention relates to an image forming device and an image forming method by which image data are transferred to a recording engine after the image data having been compressed and decompressed.
In laser printers, the laser beam modulated based on the image data is made to irradiate a polygon mirror that is driven to rotate at constant rate and the laser beam reflected from the polygon mirror is made to raster-scan a photosensitive body to form an electrostatic latent image on the photosensitive body. Therefore, it is necessary to supply image data that correspond to a raster to the laser scanner unit (to be referred to simply as “laser unit” hereinafter) disposed in the laser engine, that is a recording engine of the laser printer, in synchronism with the rotary motion of the polygon mirror that is rotated at a rate determined as a function of the printing speed (the number of sheets printed in a unit time) and the resolution (the number of dots printed on a unit length) within a predetermined period of time (printing time for a raster).
In recent years, the amount of data that needs to be supplied to the laser unit within a unit time has been remarkably increased for the purpose of realizing color printing and high speed printing. However, it is not possible to transfer the image data stored in the RAM (memory) to some other part at a sufficiently high rate because of the restrictions imposed on the bus width and the bus cycle of the RAM arranged on the main substrate of the laser printer, which is adapted to receive printing data (original data) from the host computer, convert them into image data and subsequently supply them to the laser unit. Therefore, unless the RAM is replaced by some other one that affords a greater bus width and a shorter bus cycle and hence is more costly, it is no longer possible to supply image data by the amount required for the laser unit from the main substrate to the laser unit within the time for printing a raster. Then, the laser printer can become overloaded. To avoid this problem, there has been proposed a technique with which the image data are compressed before they are stored in the RAM and transferred to some other part (e.g., ASIC (application specific integrated circuit) and subsequently the compressed image data are decompressed (expanded) so that the decompressed image data are supplied to the laser unit.
Japanese Patent Application Laid-Open Publication No. 10-16322 discloses such a technique. According to the publication, image data are converted into intermediate data that can be used to form an image on a band by band basis. When the processing time period necessary for forming the image and estimated from the intermediate data is greater than a predetermined period of time, the image data of a page (bit map data) obtained from the intermediate data are compressed and then stored in a RAM so that the image can be formed from the stored image data while the data are being expanded. On the other hand, if the processing time period necessary for forming the image and estimated from the intermediate data is smaller than the predetermined period of time, the image is formed from the intermediate data while the data are being converted into image data on a band by band basis.
According to the printing process described in the publication, the image data obtained from intermediate data are always compressed regardless of the properties and quantities of the image data, if the processing time period necessary for forming the image and estimated from the intermediate data is greater than the predetermined period of time. However, the data volume that can be reduced by compression depends on the original data volume. For instance, the data volume of a long raster in the image data can be reduced by a relatively large amount. On the other hand, the amount of data that can be reduced for a short raster in the image data by compression is relatively small and, in some cases, the data volume may conversely be increased because a coding table must be formed as a result of data compression. If such is the case, data compression does not provide any advantage because CPU requires time period for data compressing operation. Thus, time period allotted for other processing may be decreased. Consequently, processing speed of the printer is disadvantageously reduced as a whole.